[Docs] SEP-2596: Adopt MCP feature lifecycle policy; document PHP SDK BC promise mirroring 12-month spec window

[chr-hertel]

Implements SEP-2596 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #339.

Spec summary

Establishes formal Active → Deprecated → Removed lifecycle for spec features with a minimum 12-month window between deprecation and earliest removal, mandatory @deprecated schema annotations + changelog entries, and a two-SEP procedure (deprecation and removal each get their own SEP).

PHP SDK changes

• Adopt the same lifecycle policy in the SDK: PHP @deprecated tags must mirror spec-level @deprecated annotations, with the spec version that introduced deprecation noted (e.g., @deprecated Since spec 2026-07-28; will be removed no earlier than 2027-07-28).
• Audit any informal deprecations in Schema/ (e.g., legacy HTTP+SSE transport, includeContext: thisServer|allServers enum values) and document their lifecycle status.
• Establish an internal SDK policy doc — likely docs/deprecation-policy.md or BC_PROMISE.md — anchoring the PHP SDK BC promise to the 12-month spec window. Aligns naturally with Symfony's BC promise.
• Add a CI lint / static-analysis check that every spec-level deprecated element has a corresponding PHP @deprecated tag.

Related

• Umbrella [2026-07-28] Governance: deprecations, lifecycle, conformance, SDK tier #339
• SEP-2577 (companion deprecation pass)